Air compressor is a thermal machine used for repeated compression of gases known as atmospheric air to produce high-pressure air.
Air Composition: mainly 23% oxygen and 76% nitrogen by mass along (with small quantities of gases such as carbon dioxide, argon, helium, neon and water vapour)
So, air is a near perfect gas; a mixture of them will behave as a perfect gas, following Boyle’s and Charles’ laws. When air compressed its temperature and pressure increase as it volume is reduced.
The volumetric efficiency of an air compressor is measured by the number of cubic metres of free air discharged (FAD) per minute compared with the displacement of the LP piston in cubic metres per minute Modern air compressors have a volumetric efficiency of between 80 to 90%
Factors affecting Volumetric Efficiency
- Excessive bumping clearance
- Defective Valves
- Piston/Ring leakage
- Restriction of Inter/after coolers
- Restriction of Air Intake Filters
- Abnormally Hi air inlet temperature
- Restriction in discharge lines
Uses of Compressed Air
- Engine Starting
- Utilities/ Service
Compressed air is used onboard ship for a number of purposes and at varying pressures depending on that purpose.
- High-pressure air – 25 to 40 bar – starting and reversing of diesel engines by two or three stage-reciprocating units.
- Medium pressure air – 7 bar – for general service air, deck air pneumatic systems, power positioners, servo mechanisms and air puff soot blowers by single or two stage-reciprocating units.
- Medium/low pressure air – 4 and 5 bar – ‘pneu-press’ and ‘grinell sprinkler’ by single or two stage-reciprocating units or by rotary vane units.
- Low-pressure air – 2 bar – pneumatic Instrumentation/control systems by single stage or rotary vane units
1.Starting operation (average 25 to 40 Bar)
- Starting the compression ignition diesel engine required total capacity normally 30 Bar or related to it requirement with total capacity of the air bottle must generally be sufficient to start 12 times consecutively of a reversible engine and 6 time for non reversible engine.
- Auxiliary engines’ starting air bottle is usually connected to emergency air compressor driven by a small engine or manually operated
- Prior to starting and after finished with engines, blow down or blow through is done to remove any condensate or residues in cylinder through opened indicator cock in order to avoid damage due to hydraulic lock
2.Controller – Control Air (region1.4 to 7 Bar)
- Pneumatically controlled controller or instruments such as pressure, temperature, level, speed, flow etc.,- working pressure usually fall into the region1.4 to 7 Bar.
- Variety of pressures are obtained by using a small reducing valve or solenoid valve to supply air at the correct value
3.General usage (4 to 7 Bar)
- Many kind of tools are operated by compressed air – drilling machines, impact wrenches, hand grinders, lifting gears etc.,
- Auxiliary or deck machinery operated by compressed – Wilden pump, lifeboat davit etc.,
- Hydraulic jack for even tightening of the studs
- Fuel injector testing
- Water Pressure Test too.
- Compressed air widely used for cleaning purposes such as dust and cleaning at the narrow spaces where brush and finger tips cannot reach
- Cleaning of electrical machinery e.g. alternator, motor etc.,
Compressors can be classified according to the principle of compression:
- Positive displacement compressor
- Dynamic compressor
Compressors used for specific purpose are further defined by:
- The number of compression stages
- Method of cooling (air, water, oil)
- Drive method (motor, engine, steam, other)
- How they are lubricated (oil, oil-free)
- Packaged or custom-built
Read the: Type of Compressor Used in Ships
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